1. Field of the Disclosure
The present disclosure relates to an organic light emitting device and a display panel using the same. More specifically, the present disclosure relates to an organic light emitting device and a display panel using the same, to reduce a driving voltage.
2. Discussion of the Related Art
With recent trends toward large-area displays, there has been increased demand for flat display devices that take up little space. In particular, technology of organic electroluminescent (EL) devices (also called ‘organic light emitting diodes (OLEDs)’) as flat display devices has been rapidly developed. A variety of prototypes of organic electroluminescent (EL) devices have been reported to date.
When charge carriers are injected into an organic film formed between an electron injection electrode (cathode) and a hole injection electrode (anode) of an organic electroluminescent device, electrons combine with holes to create electron-hole pairs, which then decay to emit light. Organic electroluminescent devices have advantages in that they can be fabricated on flexible transparent substrates (e.g., plastic substrates) and can be operated at a voltage (e.g., 10V or below) lower than voltages required to operate plasma display panels (PDPs) and inorganic electroluminescent devices. Other advantages of organic electroluminescent devices are relatively low power consumption and excellent color representation. Further, since organic electroluminescent (EL) devices can emit light of three colors (i.e., green, blue and red), they have been the focus of intense interest lately as next-generation display devices capable of producing images of various colors. A general method for fabricating organic EL devices will be briefly explained below.
(1) First, a transparent substrate is covered with an anode material. Indium tin oxide (ITO) is generally used as the anode material.
(2) A hole injection layer (HIL) is formed to a thickness of 10 to 30 nm on the anode. Copper (II) phthalocyanine (CuPc) is mainly used as a material of the hole injection layer.
(3) A hole transport layer (HTL) is introduced into the resulting structure. The hole transport layer is formed by depositing 4,4′-bis[N-(1-naphthyl)-N-phenylamino]-biphenyl (NPB) to a thickness of about 30 to about 60 nm on the hole injection layer.
(4) An organic light-emitting layer is formed on the hole transport layer. If necessary, a dopant may be added to a material for the organic light-emitting layer. For green light emission, tris(8-hydroxyquinoline)aluminum (Alq3) as a material for the organic light-emitting layer is deposited to a thickness of about 30 to about 60 nm on the hole transport layer, and N-methylquinacridone (MQD) is mainly used as the dopant.
(5) An electron transport layer (ETL) and an electron injection layer (EIL) are sequentially formed on the organic light-emitting layer. Alternatively, an electron injection/transport layer is formed on the organic light-emitting layer. In the case of green light emission, since Alq3 has excellent electron-transport ability, the formation of the electron injection/transport layer may be unnecessary.
(6) A cathode material is coated on the electron injection layer, and finally a passivation film is covered thereon.
The type of the organic electroluminescent devices (i.e. blue, green and red light-emitting devices) will be determined depending on the kind of materials for the light-emitting layer.
Meanwhile, DNTPD, IDE406 and CuPc shown in FIG. 1 are used as materials for the hole injection layer. The materials for the hole injection layer disadvantageously cause an increase in driving voltage due to difficulty of injection of holes caused by difference in highest occupied molecular orbital (HOMO) energy level between the hole injection layer and the hole transport layer.